The present invention relates to casing paper, which may be used for the production of packaging for meat products such as sausage.
Casing paper is commonly manufactured from paper webs of relatively strong, high-tenacity natural fibers, such as abaca, sisal or flax. The paper web is saturated with a dilute viscose solution, for example, a solution obtained by diluting a solution containing 7% by weight of cellulose (as cellulose xanthate) and 6% by weight of sodium hydroxide to a 1% cellulose content. The dilute viscose-saturated web is dried and the cellulose in the viscose is then regenerated by passing the web through an acidic regenerating bath containing, for example, a 1-8% aqueous sulphuric acid solution. The web is then washed free of acid and dried to produce a paper web impregnated with acid-regenerated cellulose. This casing paper is then generally formed into rolls ("master rolls").
Casings for the packaging of processed meats, e.g. sausage, may be manufactured from the casing paper by cutting it into strips which are then folded to form tubes. The tubes are saturated with an alkaline viscose solution, containing, for example, 7% by weight of cellulose and 6% by weight of sodium hydroxide. The cellulose in the viscose is then regenerated by means of an acidic regenerating bath containing, for example, dilute sulphuric acid and possibly such salts as sodium sulphate or ammonium sulphate. The tube is then passed through one or more baths in order to wash out the acid and the salts.
If desired, the tube may be passed through an aqueous bath which contains a plasticizer, e.g. glycerine, for the regenerated cellulose. The tube is dried by passing it through a heated chamber (the tube being in an inflated state) to give a cellulosic tubing which has embedded therein a paper web. This tubing may then be stuffed with a processed meat product under pressure. A process of this type is described in detail in U.S. Pat. No. 3,135,613 and its disclosure is incorporated herein by reference.
The purpose in treating the initial paper web with the dilute (1%) viscose solution, followed by regeneration, is to provide the web with strength and structural integrity so that it may withstand the treatment with the highly caustic viscose solution used in the formation of the casing tubes. The amount of regenerated cellulose in the casing paper prior to casing formation is, in fact, comparatively low, i.e. about 6% and less by weight and preferably about 2-3% by weight. Thus, the casing paper may have a typical basis weight (weight per unit area) of 20 grams/m.sup.2, of which the regenerated cellulose accounts for 0.6 g/m.sup.2, compared with the material of the casing tubes which may have a typical basis weight of 70-80 g/m.sup.2, of which 50-60 g/m.sup.2 may be accounted for by the cellulose. However, despite the initial treatment with dilute viscose, the treatment with the highly caustic, more concentrated viscose solution used in the formation of the casing tubes will inevitably entail a certain degree of softening and weakening of the web. This imposes a limit on the production speeds if difficulty in handling the webs and possible breakdowns in production are to be avoided. There is accordingly a need in the art for casing paper having an improved alkali resistance in order to permit higher production speeds in the manufacture of the casing tubes.
In U.S. Pat. No. 3,378,379, there is disclosed a tubular regenerated-cellulose casing for dry sausage, which casing is provided with a coating comprising a cationic thermosetting resin bonded to the inside wall thereof. The patent suggests that polyethylene imine may also be employed for this coating, although this material is not, in fact, a cationic thermosetting resin. The purpose of the inner coating is to improve the adhesion of the sausage casing to a dry sausage product despite any shrinkage which may occur when the dry sausage product is processed and dried in the casing over a prolonged period of time. It should be noted, however, that in the aforesaid process it is not the casing paper as such which is treated with the thermosetting resin, but the tubular casing material. In the embodiment illustrated in U.S. Pat. No. 3,378,379, the cationic thermosetting resin is applied to the inner surface of the casing tube after the application of glycerine and before the casing is dried, in an inflated state, in a heated chamber.
Another problem recognized in U.S. Pat. No. 3,378,379 is the variation in extensibility in the transverse direction exhibited by casing paper strips cut from different parts of the master roll. This can cause variation in the properties of the final casing tubes, which may therefore be unsatisfactory to the meat packager, for whom dimensional stability in the product is of commercial importance. To meet this problem it is suggested in the aforesaid U.S. Patent that a cationic thermosetting resin (e.g., a reaction product of epichlorohydrin and a polyamide, a modified melamineformaldehyde resin or a modified urea-formaldehyde resin) may be employed as the bonding agent in the casing paper, instead of the commonly employed acid-regenerated viscose. The cationic thermosetting resin is employed in an amount of at least 0.5% by weight based upon the dry weight of the impregnated fibrous web. The resin may be incorporated into the fibrous web by adding the resin to the fibrous slurry prior to forming the fibrous web. Alternatively, the formed fibrous web may be impregnated with the resin by passage through an aqueous solution of said resin. The said U.S. Patent also discloses the use of the thermosetting resin in combination with viscose which is not regenerated with an acid; however, in the latter case, the viscose may be auto-regenerated by storing the web for a sufficient period.
In United Kingdom Pat. No. 1,091,105, there is described a process in which casing paper is produced by incorporating into a paper web an alkaline-curing resin such as polyethylene imine or a polymeric reaction product of epichlorohydrin and a polyamide. The use of the alkaline-curing resin in place of the customary treatment with dilute viscose is said to result in a casing paper which has more uniform characteristics across its width and which results in a casing having improved burst strengths. However, it has now been found that the casing paper so produced has an alkali resistance which is insufficient to permit sufficiently high production speeds for the commercial manufacture of the casing tubes.
It is an object of the present invention to provide a process for the production of casing paper having a good alkaline wet strength and from which casing tubes may be produced using high-speed commercial processes.
Other objects will be in part obvious and in part pointed out more in detail hereinafter.
Accordingly, the present invention provides a process for the production of casing paper of improved alkaline wet tensile strength suited for subsequent converstion into casing, which process comprises forming a fibrous paper web, treating the fibrous paper web with a dilute viscose solution and subsequently regenerating the cellulose in the viscose by means of an acid and treating the fibres of the paper web with a cationic polyethylene imine resin. Preferably, the resin treatment is carried out prior to, or simultaneously with, the treatment with the said dilute viscose solution.
The present invention also provides casing paper when prepared by the aforesaid process, as well as casing material (casing tubes or skins) prepared from the said casing paper by a process comprising the application of a caustic viscose solution and subsequent acid regeneration.
A better understanding of this invention will be obtained from the following description of the process including the several steps and the relation of one or more of such steps with respect to each of the others and the article of manufacture possessing the features, characteristics, compositions, properties and relation of elements described and exemplified herein.